As an art for forming a semiconductor element on an insulating substrate, the following art has been studied recently; a transistor (such as a semiconductor transistor) composed of a monocrystalline silicon thin film (hereafter, referred to as “monocrystalline Si thin film”) is formed on a semiconductor substrate in advance, then the transistor is bonded onto a (insulating) substrate on which the transistor is finally mounted, which substrate is a so-called production substrate and is a glass substrate etc., and the semiconductor substrate is then cleaved and stripped off to be separated from the transistor thereby attaining a thin film (see, for example, Patent Documents 1 and 2 and Non-Patent Document 1).
Especially, for the sake of easy microfabrication of monocrystalline silicon, a transistor composed of a monocrystalline silicon thin film (hereafter, referred to as “monocrystalline Si transistor”) is preferably formed by performing on a monocrystalline Si substrate (monocrystalline Si wafer) formation of a gate electrode and impurity ion doping for forming a source/drain, compared to performing the formation of the transistor after forming a monocrystalline Si thin film on a production substrate.
The monocrystalline Si transistor is initially formed on a monocrystalline Si substrate. Then the monocrystalline Si transistor formed on the monocrystalline Si substrate is bonded onto the production substrate, in such a manner that the monocrystalline Si substrate is on the outer side. Then, the monocrystalline Si substrate is stripped off from the monocrystalline Si transistor. In this way, the monocrystalline Si transistor is transferred from the monocrystalline Si substrate onto the production substrate.
As for a method for stripping the monocrystalline Si substrate off from the monocrystalline Si transistor bonded onto the production substrate, the so-called Smart-Cut® method is generally employed. According to the Smart-Cut method, doping a predetermined concentration of hydrogen ions or noble gas ions into the monocrystalline Si substrate causes microbubbles in a doping section of the monocrystalline Si substrate. The monocrystalline Si substrate is cleaved at a layer of the microbubbles, utilizing the layer as a cleaved surface (see Patent Documents 1 and 2).
More specifically, after forming the monocrystalline Si transistor on the monocrystalline Si substrate, hydrogen ions are doped into the monocrystalline Si substrate, thereby forming a hydrogen ion doping layer with a predetermined depth from the surface of the monocrystalline Si substrate. Then, the monocrystalline Si substrate is bonded onto the production substrate and is heat-treated. As a result, the monocrystalline Si substrate is separated at the hydrogen ion doping layer, thereafter being stripped from the monocrystalline Si transistor.